Supporters of the Workers’ party candidate, Fernando Haddad, argue with supporters of the far-right candidate Jair Bolsonaro after the results of the elections, in Rio de Janeiro.
Photograph: Julio Cesar Guimaraes/EPA

Opponents of Brazil’s newly elected far-right president, Jair Bolsonaro, have vowed to hit the streets to oppose the intolerance and violence they fear he will inflict upon the world’s fourth largest democracy.

The once inconceivable election of the 63-year-old populist provocateur on Sunday represents a hammer blow to Brazil’s left and to millions of progressive Brazilians appalled by his hostility towards black, gay and indigenous people as well as the environment and human rights.

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“Never have we sunk so low, never have we been so repulsive to the world,” one prominent leftwing commentator wrote in Brazil’s Folha de São Paulo on Monday in an article headlined “Rock Bottom”.

But as Bolsonaro’s stunning triumph sunk in, leftwing leaders insisted they would not be cowed.

Guilherme Boulos, a 36-year-old social activist, told the Guardian: “There will be resistance, there will be opposition, there will be street mobilizations. Our voices will not be silenced.

“Bolsonaro won the election by exploiting people’s fears and their disillusionment with politics. But the fact that he won the election does not mean he owns the country,” added Boulos, who ran for the presidency for the Socialism and Liberty party (PSOL).

“Democracy is at real risk so we will build a democratic resistance … One week ago Bolsonaro said that anyone who opposed him could choose between prison or exile. We choose the streets,” he vowed.

Bolsonaro does not take power until 1 January, but the first protests against his administration are set to take place on Tuesday afternoon in cities across the country.

One will be held outside São Paulo’s brutalist art museum, the scene of wild pro-Bolsonaro celebrations on Sunday night. Another is expected in downtown Rio de Janeiro.

“My message is: ‘Resist!’ There is no reason to be afraid,” Boulos said. “We must show courage and serenity and remember that the defense of democracy is always the right side of history.”

In an online video message, Boulos told supporters the “dark clouds of intolerance and violence” hanging over Brazil would clear sooner than they expected.

Marcelo Freixo, a leading PSOL politician, said progressive politicians from both left and centre needed to form a pro-democracy coalition that could push back against Bolsonaro’s authoritarian tendencies in Congress. “He was elected democratically. This has to be recognised. But it is his obligation to preserve democracy and we have to mobilise to make sure this happens.”

Freixo, a federal deputy and veteran human rights activist in Rio de Janeiro, said one particular concern was Bolsonaro’s pledge to criminalise social movements. “Bolsonaro has clearly threatened activism. He has said he will put an end to activism … We cannot sit around waiting for this to happen.”

Speaking on the eve of the election, Workers’ party (PT) veteran Aloízio Mercadante pledged his party would also push back, shrugging off Bolsonaro’s pre-election threat that “red” opponents would be jailed or forced into exile.

“I didn’t leave during the dictatorship … I resisted. And I will carry on resisting,” said Mercadante, Brazil’s former chief of staff.

On Sunday night, Fernando Haddad, the defeated PT candidate, vowed to help spearhead the resistance movement to Bolsonaro. “I put my life at the service of this country.”

Elated devotees of Bolsonaro, who secured nearly 58 million votes compared with Haddad’s 47 million, continued commemorating his victory into the early hours of Monday.

“We’ll make the Americas great again,” tweeted Filipe Martins, a rightwing academic who backed Bolsonaro’s campaign.

“We think he can get the train back on the tracks,” said Iago Bünger, a 19-year-old student who was among thousands of supporters to converge on São Paulo’s Avenida Paulista for commemorations.

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But Bünger also expressed nervousness about the future. “Sincerely, I have my uncertainties. Bolsonaro is an unknown quantity.”

“It’s an inflection point,” his friend, Luiz Guilherme Bassi, 16, said. “Might it go wrong? Yes. But at least he has the right ideas.”

There was uncertainty, too, in Brazil’s press.

Writing in the Folha de São Paulo, the political commentator Celso Rocha de Barros argued: “The catastrophe that struck Brazil on Sunday cannot be downplayed … We have the most extremist leader of all democratic nations.”

Even the conservative Estado de São Paulo, whose coverage has been considerably less combative towards Bolsonaro, expressed unease at what it called Brazil’s “leap into the dark”.

“Voters have chosen Bolsonaro without having the slightest idea of what he will do when he sits down in the presidential chair,” it warned in a front-page editorial.

The newspaper lamented how the mission of extricating Brazil from profound political and economic crisis now fell to a man famed for his “hateful, empty” rhetoric. “This does not bode well, precisely at the moment when the country needs clarity, competence and leadership.”